(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular, to a liquid crystal display device which is driven by a so-called in-plane switching system.
(2) Description of the Related Art
In liquid crystal display devices, the space between a TFT substrate on which pixel electrodes and thin film transistors (TFTs) are formed and the facing substrate on which color filters are formed is filled in with liquid crystal so that the molecules of this liquid crystal are driven and controlled through an electrical field so as to form an image. From among them, liquid crystal display devices driven by a system that is referred to as an in-plane switching system (IPS system) have been widely used in recent years.
The IPS system is a system for driving liquid crystal where liquid crystal molecules are aligned horizontally to the surface of the panel and are rotated within a plane parallel to the surface of the panel by applying an electrical field (lateral electrical field) parallel to the surface of the panel. In the liquid crystal display devices of this IPS system, common electrodes are also formed on the first substrate where video signal lines (drain lines), scan signal lines (gate lines), thin film transistors and pixel electrodes are formed, and the liquid crystal layer is driven by an electrical field in a direction in the plane of the first substrate that is created as the difference in the voltage applied to the pixel electrode and the common electrode. In the liquid crystal display devices of the IPS system having this structure, linear pixel electrodes are aligned in a layer above the common electrodes in sheet form formed of a transparent conductive film, for example, with an insulating film in between, so as to overlap the common electrodes.
A type of this IPS system where wall structures are formed so as to cross adjacent pixels in the liquid crystal display device in order to increase the aperture ratio in the liquid crystal display unit, pixel electrodes are formed on the sidewalls of these wall structures, and common electrodes and counter electrodes are formed on the TFT substrate and the facing substrate, respectively, so that electrical fields parallel to the substrate surface are generated in order to drive the liquid crystal layer has been widely used in recent years.
In addition, the gap between the TFT substrate and the facing substrate in the liquid crystal display devices is several microns, which is very small, and therefore, it is extremely important to set the gap between the TFT substrate and the facing substrate to an appropriate value in order to control the transmission of light through the liquid crystal. Therefore, it has been proposed to allow the wall structures to also function as spacers for maintaining the gap between the TFT substrate and the facing substrate.
When a liquid crystal display device is manufactured, it is necessary to fill the space between the substrates with liquid crystal and seal it. The methods for filling the space with liquid crystal that have been widely used in recent years include a method that is referred to as the liquid crystal drop filling method (ODF system), according to which a required amount of liquid crystal is first dropped on one substrate, and after that is sealed with the other substrate, and thus, the space between the substrates is filled in with liquid crystal.
Though the ODF system has such advantages that large scale manufacturing facilities as compared to those using a conventional liquid crystal filling method are unnecessary, the time for manufacture can be shortened and mass production is easy, very high precision is required in the dropping of the liquid crystal and for the maintenance of the gap between the substrates.
When this ODF system is applied to the liquid crystal display device where the wall structures function as a spacer as described above, there is a concern about the generation of low temperature impact bubbles. The low temperature impact bubbles are bubbles generated in an environment where the temperature is as low as approximately −20° C., particularly from among so-called vacuum bubbles that are generated when an impact due to an external force is applied to the liquid crystal panel where liquid crystal is sealed and a negative pressure is created in the liquid crystal layer, and thus, a gas component, such as of nitrogen, that has been dissolved in the liquid crystal layer comes out.
It is difficult for these low temperature impact bubbles to be dissolved again, and thus, it is not easy for them to disappear, thereby often causing inconsistency in the display. Low temperature impact bubbles are easily generated in the portions where a substrate and a spacer make contact with each other, and it has been experimentally confirmed that the measures for preventing low temperature impact bubbles from being generated perform inversely proportional to the area of contact between the spacers and the substrates.
The reason why low temperature impact bubbles are generated, which is a problem, in the liquid crystal display devices of the IPS system using wall structures is that the height of the wall structures is constant, and the wall structures are formed along all of the long sides of the pixels, and therefore, the liquid crystal display devices are in such a state where only so-called main spacers are aligned with high density, which provides no space where sub-spacers that are slightly shorter than the main spacers are arranged. That is to say, the area of contact between the substrates and the spacers is large as compared to a case where the sub-spacers are arranged.
In the case when a liquid crystal display device where the wall structures also function as spacers is manufactured using the ODF system as described above, the wall structures directly receive the pressure when the substrates are pasted together, and thus, there is a concern that the ITO, which is electrodes, the interlayer insulating films and the wall structures may be damaged.
JP 2005-157224A discloses a technology according to which wall structures and supports are arranged between substrates so that the thickness of the liquid crystal layer is controlled in a liquid crystal display device of the VA system. JP 2009-145865A and JP 2010-210866A also disclose a technology for maintaining the thickness of the space between the substrates using spacers. However, all of these are insufficient in preventing low temperature impact bubbles from being generated in the IPS system or in preventing the wall structures from being damaged.